The plot of High and Low in the manga. The president of a company had an aneurysm, and although Black Jack agreed to operate on him for 50 million yen, he had a rare blood type (RH-) which sent his men on a frantic search for a suitable donor. Fortunately, a laborer nearby was found to be the same blood type, and the operation went without a hitch. The two men bonded greatly over this event. One day, the laborer had an accident while the president was about to leave on a plane to meet a very important client. After a little internal conflict, the president decided to ditch the flight and give blood to save the laborer's life. Unfortunately, losing such an important client resulted in his company going bankrupt in short order. The laborer found the now-penniless president and told him not to lose heart, and that he will help the president in starting over. Black Jack then gave the men an envelope with a check of 49.9 million yen, as he "owed the president his change from the operation back then". The last panel showed the 2 men walking together, ready to start anew.
The chapter "Fake Doctor", where a kid gets another kid that he formerly bullied, who has parents that are actors, to play a fake Black Jack for his sick sister (who wants only Black Jack to diagnose/operate on her). Of course the real Black Jack shows up at the end and takes it from there, but it's adorable.
Episode 45 of the TV Series, "The Chronically Laughing Classmate" is a whole episode flashback of one of Kuro Hazama's inspirations to practice medicine. An Anti-Nihilist classmate taught him to start smiling again and later even saved his life. A reason Black Jack went into medicine was because the injury the classmate sustained ruined his vocal chords and made it hard to laugh... and Black Jack later was finally able to repay him by restoring his ability to laugh in an operation. Of course, it goes into Tear Jerker territory when the man dies from a serious case of pnuemonia, but...
"His laughter was very bright and it echoed very far. It was even audible outside of the building, and even stopped the birds' chirping for a moment. I've never heard such great, honest laughter before."